Composite materials (hereinafter “composites”) are currently used as a replacement for metallic materials in many industrial applications because of their resistance to corrosion. In the oil and gas industry for instance, composites are used in filament wound composite structures such as pipes and vessel tanks. Examples of such composites include reinforced thermosetting resin (RTR) pipe (fiberglass), fiber-reinforced-polymer (FRP) systems, glass reinforced polymer (GRP) and glass reinforced epoxy (GRE). A cross-section and side view of part of a pipe made from a composite material is shown in FIGS. 14A and 14B. While use of composites is prevalent in low-pressure hydrocarbon applications, there has been continued resistance in employing composite in high-pressure applications due to the difficulty in monitoring structural integrity.
Given the susceptibilities of composites to certain types of damage, particularly in high-pressure applications, it is important to periodically inspect composites to test whether such damage has occurred or is accumulating. It is also a requirement for the inspection to be non-destructive because it is infeasible to employ invasive techniques that interrupt the continued operation of the structures in the field. Suitable non-destructive testing (NDT) techniques should be able to accurately detect typical defects in composites, be easy to apply, and permit rapid and automated inspection of large areas. It would also be advantageous for such techniques to provide in-service inspection with minimum surface preparation.
Among common NDT techniques, infrared thermography stands out as a good candidate since it provides contact-free measurement (no need for coupling media), global and focused area scans, fast acquisition, and easy operation. Limitations of the sensitivity of infrared thermography equipment have until now restricted this technique to qualitative and boundary inspections, both of which are unable to provide accurate defect size, depth data or data on the nature of any entrapped media, and are limited to detecting defects located close to the surfaces of the inspected structures.
There is therefore a need for non-destructive techniques for rapidly, reliably and cost-efficiently inspecting composite structures in an accurate quantitative manner. The present invention is addressed to this and related needs.